1. Field of the Invention
The present invention relates to a color image forming apparatus for forming an image by an electrophotographic process such as a copying machine, a printer, a facsimile apparatus or the like.
2. Related Background Art
In the conventional image forming apparatus capable of forming a color image by an electrophotographic process, the image bearing member supporting toner on an electrostatic latent image is generally composed of a metal drum or a metal belt provided with a photosensitive layer composed of a charge generating layer and a charge transporting layer.
In such image forming apparatus, the image bearing member is driven in a predetermined direction in response to a print start signal and then is given a bias by a charging device (a corona charger, a charging roller or a member bearing magnetic particles or the like) whereby the surface of the image bearing member is charged or is subjected to charge injection to a predetermined potential.
The surface potential thus obtained is called a VD potential. Then the surface of the image bearing member is irradiated with light of a laser or a light emitting diode (LED) which is on-off controlled according to a signal from a controller (exposure). The potential is reduced in a portion irradiated with the light, whereby an electrostatic latent image is formed on the surface of the image bearing member. The potential of thus exposed portion is called a VL potential.
Then a bias (developing bias) is applied to a developing device opposed to the image bearing member and filled with toner to transfer toner bearing predetermined charge onto the electrostatic latent image on the image bearing member, thereby rendering the electrostatic latent image visible (development). The toner amount deposited on the image bearing member at the development is determined by the relationship between the aforementioned VL potential and the developing bias.
Then a bias of a polarity opposite to that of the toner on the image bearing member is applied to a transfer member, such as a transfer roller or a transfer belt, positioned adjacent to the image bearing member and moved in the same direction and in a substantially same speed as those of the image bearing member, and a transfer material is passed in such state between the transfer member and the image bearing member, whereby the toner carried on the image bearing member is transferred onto the transfer material.
In the image forming operation explained in the foregoing, the photosensitive layer of the image bearing member is affected not a little by the exposure memory effect by the irradiating light. More specifically, the VL potential after the exposure fluctuates by the history of use of the image bearing member. Such fluctuation leads to a fluctuation in the deposited toner amount, whereby the density of the printed image cannot be made constant.
In order to avoid such exposure memory effect, there is conventionally adopted a method of providing a suitable light source (pre-exposure light source) in the apparatus and executing so-called pre-exposure in which the surface of the image bearing member is exposed to the light from the pre-exposure light source prior to the charging operation of the image bearing member. However, such light source (pre-exposure light source) may not be provided in the apparatus as a result of recent compactization of the apparatus.
For this reasons, for realizing a compact image forming apparatus without the pre-exposure device but provided at least with a movable electrophotographic photoreceptor, charging means for charging the surface of the electrophotographic photoreceptor and exposure means for exposing the surface of the electrophotographic photoreceptor charged by the charging means thereby forming an electrostatic latent image, there is being proposed a method of taking a potential difference obtained in an area, where the potential attenuation in an exposed portion by a change of exposure amount of 0.1 (xcexcJ/cm2) with respect to an arbitrary surface potential obtained by charging the photoreceptor becomes 50 V or less, as the maximum latent image contrast potential difference for such arbitrary surface potential, and selecting the image forming condition such as the developing potential within a range of the exposure amount where the potential difference becomes 60% to 90% of such maximum latent image contrast potential difference.
However, the above-described method of reducing the exposure amount to reduce the latent image contrast potential difference thereby correcting the developing condition such as the charging potential of the photoreceptor or the developing bias potential same as in the ordinary image forming condition allows to obtain a maximum density of the image forming apparatus same as in the normal condition, but, since the image formation is executed under a condition where the relationship between the charging potential of the photoreceptor and the exposure amount (E-V characteristics) is different from the normal, there will result a change in the intermediate density due to a change in the potential of the intermediate density area caused by a difference in the potential attenuation with respect to the exposure amount.
An object of the present invention is to provide a color image forming apparatus and a control method therefor, free from the aforementioned drawbacks.
Another object of the present invention is to provide an image forming apparatus and a control method therefor, capable of varying the density of the output image involving the intermediate density according to the difference in the potential attenuation, thereby providing image quality same as in the normal condition.
Still another object of the present invention is to provide an image forming apparatus and a control method therefor, capable of automatically correcting the intermediate density area so as to constantly providing a high quality image without the toil of the user.
Still other objects of the present invention, and the features thereof, will become fully apparent from the following description to be taken in conjunction with the accompanying drawings and from the appended claims.